Portable, hand-held radio tracking receivers are commonly utilized in tracking systems of different kinds. FIG. 1a depicts one example of a tracking system known in the art. In the depicted tracking system, a target 2, in this example a dog, to be tracked carries a radio transmitter 4. The radio transmitter comprises an omnidirectional antenna that sends radio signals 6. The radio transmitter 4 may transmit periodically to save limited battery power of the radio transmitter 4. The transmitter may for example transmit a short radio pulse once in a second. The depicted exemplary tracking system also comprises a directional radio receiver 5 of the tracker 1. The receiver 5 is sometimes also called a radio-bearing device. The tracker 1 uses the radio-bearing device 5 to determine from which direction the radio signals 6 are coming. After determination it is known in which direction the target 2 is. Radio tracking systems are used, among other things, in the positioning of pets and hunting animals.
The ability of a radio-bearing device 5 to determine the incoming direction of the radio signal 6, i.e. take a bearing, is based on a directional antenna 51 of the radio-bearing device 5. A directional antenna 51 has the basic property that it receives radio signals well when a beam of the antenna 51 is directed straight towards the radio transmitter 4 sending radio signals 6. The directional antenna 51 attenuates considerably radio signals coming from other directions. The mechanical structure and dimensions of the directional antenna 51 depend on the used radio frequency. The antenna used in the mobile tracking receivers on the market today is generally a so-called Yagi antenna with 2 or 3 antenna elements. A directional antenna may be integrated into the same body with the radio-bearing device 5 in order to create an easy-to-use and small-sized hand-held radio-tracking receiver.
FIG. 1b depicts an example of an exemplary visual display unit 52 of a bearing-device 5. The depicted display unit 52 is in the example an integral part of the radio-bearing device 5 (which has not been shown completely in FIG. 1b). The exemplary display unit 52 includes a scale 53 that can be used to show received signal level or strength. In the depicted example, a pointer 54 shows on the scale 53 how strong the received signal is. In the example of FIG. 1b reading “0” on the scale 53 represents a situation where a received signal level is very low or not existing at all. On the other hand, reading “100” on the scale 53 represents a situation where a very strong received signal saturates the receiver circuit of the radio-bearing device 5. In the depicted example of FIG. 1b, the pointer 54 shows that the radio-bearing device 5 has received quite a strong signal from a direction whereto the antenna of the bearing device is directed.
When the tracker 1 turns the radio-bearing device 5 in a way where the antenna beam of the radio-bearing device 5 changes its direction, the reading 54 on the scale 53 of the display unit 52 changes. Depending on if the antenna of the radio-bearing device 5 is turned towards the transmitter device 4 or away from the transmitter device 4, the reading on the scale 53 of the display unit 52 either increases or decreases. The tracker 1 tries to find a maximum reading on the display unit 52 by turning the radio-bearing device 5 to different directions. However, when finding the direction of the maximum transmission, the tracker 1 has to remember the detected direction of the tracked target 2. If the radio-bearing device 5 is turned away from the direction of the transmitter 4 of the target 2, the reading on the display unit 52 decreases. Therefore, the tracker 1 has to remember all the time wherefrom the maximum reading has been measured.